Recently available air-conditioning systems for automotive vehicles are provided with various kinds of convenience devices for enhancing the pleasantness within a vehicle room. For example, there are known a console air-conditioning system for cooling or heating a console box and a seat air-conditioning system for cooling or heating a driver seat and a front passenger seat.
As shown in FIG. 1, the console air-conditioning system is configured to supply a cold air or a hot air generated in an air-conditioning unit 3 to a console box 1. This makes it possible for the console box 1 to serve as a refrigerator or a heating cabinet.
The seat air-conditioning system is designed to feed a cold air or a hot air generated in the air-conditioning unit 3 to a plurality of air discharge holes 8 formed in the seat portions 5a and 7a of a driver seat 5 and a front passenger seat 7. Thus, the cold air or the hot air is discharged from the air discharge holes 8 of the seat portions 5a and 7a. This makes it possible to cool the driver seat and the front passenger seat in summer, while warming them in winter.
The cold air or the hot air supplied from the air-conditioning unit 3 to the seat portions 5a and 7a is fed to the air discharge holes 8 of the seat portions 5a and 7a through different ducts D which are branched from a console duct 1a leading to the console box 1.
However, the conventional seat air-conditioning system set forth above suffers from shortage in the flow rate of the cold air or the hot air, because it makes use of a part of the cold air or the hot air drawn from the console duct 1a. This leads to a reduction in the capability of cooling or heating the seat portions 5a and 7a. 
In view of this, there have been proposed techniques in which the capability of cooling or heating the seat portions 5a and 7a is improved by increasing the flow rate of the cold air or the hot air supplied to the seat portions 5a and 7a. 
One example of these techniques is disclosed in JP2004-161061A. As shown in FIG. 2, this technique is directed to a configuration in which an auxiliary blower 3b is arranged in an air-conditioning case 3a of an air-conditioning unit 3. The auxiliary blower 3b is integrally mounted to the air-conditioning case 3a and is used to increase the quantity and pressure of a cold air or a hot air fed to a console duct 1a. This makes it possible to increase the flow rate of the cold air or the hot air supplied to the seat portions 5a and 7a of a driver seat 5 and a front passenger seat 7, thereby enhancing the cooling or heating performance for the seat portions 5a and 7a. 
Since the auxiliary blower 3b is integrally mounted to the air-conditioning unit 3, the above-cited conventional technique poses a problem in that the air-conditioning unit 3 becomes structurally complicated, costly to manufacture and difficult to fabricate.
In particular, a blower chamber 3c for accommodating the auxiliary blower 3b needs to be formed in the air-conditioning case 3a. It is also necessary to provide an air passage 3d kept in communication with the blower chamber 3c. In addition, there is a need to install a variety of doors 3e along the air passage 3d. This results in a highly complicated configuration and requires doing a difficult task when assembling different parts together.
Inasmuch as the auxiliary blower 3b is integrally mounted to the air-conditioning unit 3, the conventional technique has a drawback in that the air-conditioning unit 3 cannot be interchanged with an air-conditioning unit having no auxiliary blower, namely an air-conditioning unit having no need to cool or warm seat portions nor to increase the flow rate of a cold air or a hot air.
This means that, in case of an air-conditioning system with no function of cooling or heating seats, there is a need to additionally manufacture an air-conditioning unit having no auxiliary blower. This results in a sharp increase in the manufacturing cost.
In addition, the conventional technique has a disadvantage in that the flow rate of the cold air or the hot air supplied to the seat portions 5a and 7a is insufficient because the cold air or the hot air supplied to the seat portions 5a and 7a is diverted from the console duct 1a. This reduces the capability of cooling or warming the driver seat 5 and the front passenger seat 7.